Thicken a Surface

What are the criteria to thicken a surface? Does zebra stripes and curvature clearly tells which region needs repair to thicken? 

What is the reason sometimes surface thicken is possible only in one direction and fail in other direction (side 1 or side 2) also why thicken fails if we increase the thickening dimension.   

There are many evaluation tools and video demos which are not clear enough.

SolidworksSurfacing